Flynn's military career included a key role in shaping U.S. counterterrorism strategy and dismantling insurgent networks in Afghanistan and Iraq, and he was given numerous combat arms, conventional, and special operations senior intelligence assignments. He served as the 18th Director of the Defense Intelligence Agency, serving from July 2012 until his forced retirement from the military in August 2014. After leaving the military, he established Flynn Intel Group, which provided intelligence services for businesses and governments, including ones in Turkey. He became a senior advisor to Trump during his presidential campaign and served as the National Security Advisor from January 23 to February 13, 2017. He resigned after information surfaced that he had misled the FBI and Vice President Mike Pence about the nature and content of his communications with Russian Ambassador to the U.S. Sergey Kislyak. Flynn's tenure of just 24 days is the shortest in the office's history.
On April 27, 2017, the Pentagon inspector general announced an investigation into whether Flynn had accepted money from foreign governments without the required approval. The New York Times reported on May 18, 2018, that a longtime FBI/CIA informant had met Flynn at an intelligence seminar in Britain six months earlier and became alarmed by Flynn's closeness to a Russian woman there; this concern prompted another individual to alert American authorities that Flynn may have been compromised by Russian intelligence. Flynn initially refused to hand over subpoenaed documents to the Senate Intelligence Committee, pleading the Fifth Amendment against self-incrimination, but a compromise with the committee was worked out. On December 1, 2017, Flynn appeared in federal court to formalize a deal with Special Counsel Robert Mueller to plead guilty to a felony count of "willfully and knowingly" making "false, fictitious and fraudulent statements" to the FBI. He confirmed his intention to cooperate with the Special Counsel's investigation.